The invention relates to an abradable protective coating for blade outer air seals, and more particularly to an abradable coating that is geometrically conditioned to result in more uniform thermal heat flux regions throughout the blade outer air seal.
Blade outer air seals in the high pressure, high temperature regions within a gas turbine engine generally include some form of protective layering, such as a thermal barrier coating (TBC). These protective layers function to protect the blade outer air seals from oxidation, corrosion, and thermal-mechanical fatigue that can reduce part life and the repair-ability of engine run parts. In conventional blade outer air seals, the seals include an abradable ceramic TBC that is removed over the life of the part by the rotation of turbine blades adjacent it. The effect of turbine blade abrasion is to create a clearance pocket on the blade outer air seal radially adjacent the turbine blade tips. Abrasion of the TBC typically provides a minimum clearance between the outer air seals and the turbine blades such that gas flow around the tips of the turbines is reduced. This reduction in gas flow prevents leakage that would result in reduction of engine efficiency.
Although the pocket assists in reducing gas flow around the blade tips, some of the ancillary effects of this pocket are generally not desirable. The pocket is generally more pronounced along a turbine blade rub path that is created by rotating motion of the turbine blades relative to the static blade outer air seal. The pocket is also a region of high heat flux due to aerodynamics and less insulating material being located in that region. One drawback of the conventional pocket blade outer air seal design is that it geometrically creates a non-uniform TBC thickness. The overall effect of this non-uniformity in the TBC creates a thermal gradient that increases thermal-mechanical fatigue, and concentrates stresses in the blade outer air seals. This phenomena accelerates base metal degradation in the pocket area.
There is a need for a blade outer air seal that will result in reduced thermal gradients and reduced associated heat transfer, stress, and thermal mechanical fatigue.